Organosulfur compounds have been employed in emulsion polymerization systems as molecular weight modifiers or regulators. For a satisfactorily high level of activity, conveniently measured by the regulating index, lower molecular weight species heretofore generally have been employed, since the high molecular weight species exhibit low regulating indexes. The lower molecular weight species have higher volatilities, which frequently tend to leave undesirable readily apparent residual odors in the polymeric products. Higher molecular weight species, with lower volatilities, would be preferred, if the regulating index could be increased sufficiently.
It is known to preadmix and coagitate together an aqueous emulsifier and an organosulfur compound prior to addition of the balance of the aqueous polymerization mixture, such coagitation being a sufficiently thorough mixing of the modifier and emulsifier to afford significant reduction in the particle size of the organosulfur compound and to increase the regulating index. This has been done by such as shearing action, various mills, and the like, as described in U.S. Pat. No. 4,064,337 to Uraneck and Burleigh Dec. 20, 1977.
However, organosulfur compounds so-used have exhibited undesirable rates of loss, i.e., depletion, during the early stages of monomer conversion in the emulsion polymerization system. To alleviate this early depletion experienced in the coagitation approach, other steps have been taken. For example, a small amount of preformed polymer latex has been added to or with the emulsifier and modifier admixture, as described in U.S. Pat. No. 3,928,498 to Uraneck and Burleigh Dec. 23, 1975. Another approach has been to add during the coagitation of the modifier and emulsifier an additive such as a monomer-soluble-hydrocarbon diluent, or else a polymerizable monomer itself, as described in U.S. Pat. No. 4,013,824 to Uraneck and Burleigh, Mar. 22, 1977. Sometimes some loss of modifier has occurred during the coagitation step itself, and various reducing agents have been added to the coagitation step, such as described by Uraneck and Burleigh in U.S. Pat. No. 3,855,188 Dec. 17, 1974.
Even with the improvements attainable using these preemulsifying processes, efficient effective use of the higher molecular weight modifiers is not always achieved. This necessitates the use of undesirable amounts of modifier, at least more than should otherwise be necessary, adding to the cost of the polymer and resulting in variations in product quality. It is certainly preferable for both economy and product quality with minimum residual odor to use minimum amounts of modifier consistent with effectiveness.